Smoking is the leading cause of death and disability in the U.S. Despite this, 21% of U.S. adults continue to smoke on a regular basis and less than about 20% are ready to quit smoking or taking action to quit at any given time. In other words, about 80% or more of smokers are not sufficiently motivated and ready to quit smoking, even though they may want to quit someday. Interventions are critically needed which can reach these people, enhance their motivation for quitting, promote uptake of existing empirically-validated treatments, and ultimately enhance abstinence rates on a population level. Motivational interventions delivered via the Internet can be easily tailored to the needs of individual smokers. There is now a large body of research demonstrating that computer-tailored health behavior change interventions can be more effective than generic interventions, particularly for smoking cessation, but it is less clear what specific content and presentation factors will be most effective and for whom when tailoring an online motivational intervention. The current study will test the effectiveness of five potentially important tailoring factors (decisional framework, message framing, self-efficacy, navigation autonomy, and proactive outreach). Factor choice was largely informed by the principles of Motivational Interviewing, Social Cognitive Theory, Prospect Theory, and the Transtheoretical Model. In addition to examining whether each factor is important for treatment outcome, we will also examine for whom each factor is most important by exploring the influence of relevant individual characteristics which may mediate or moderate outcomes. This level of understanding is important for designing future tailored programs. Participants in this study will be recruited from a large, regional U.S. health plan. Using a fractional factorial design to screen for important main effects and 2-way interactions, participants will be randomized to receive one of 16 different experimental factor combinations and followed for one year to assess program impact on smokers' motivation for quitting, use of empirically-validated cessation treatment, and abstinence. The proposed research is a natural extension of our collaborative team's work in tailoring, developing motivational and action oriented programs for smoking cessation, health risk communications research, and creating online health behavior change programs. Information learned from this study will advance the science of tailoring health risk communications, as well as inform how to best design an online motivational intervention for smoking cessation. This is the first study we are aware of to address this important issue. PUBLIC HEALTH RELEVANCE: Reducing smoking prevalence in the U.S. will require motivational interventions which are both effective at promoting abstinence and can be easily disseminated on a population level. Computer-tailored programs delivered over the Internet hold great potential for this purpose, but research in this area must move beyond simply comparing tailored to non-tailored programs and begin to specify which content and presentation factors enhance tailoring effectiveness and for whom. The proposed study is the first we are aware of to examine this issue in the context of an online motivational intervention for smoking cessation. Information learned from this study will advance the science of tailoring health risk communications, as well as inform how to best design an online motivational intervention for smoking cessation.